1. Field of the Invention
The present invention relates to a method and apparatus for boosting the pressure of the air charge to an internal combustion (IC) engine which has an exhaust gas recirculation (EGR) system, and in particular to such an engine having a combined EGR/turbocharger bypass control valve for controlling the amount of recirculated exhaust gas and the power of a turbocharger compressor. The invention is particularly although not exclusively for use in a diesel engine.
2. Related Technology
One way of boosting the pressure of the air charge to an IC engine is to use a turbocharger, which typically has a rotary compressor for compressing the intake air driven by a turbine wheel powered by engine exhaust gas. The result is that more air is sent into an engine's combustion chamber, increasing engine power.
When air is compressed, it is simultaneously heated. So, compressing engine intake air raises the temperature of air sent into the engine's combustion chamber. The raised air temperature increases the temperature of the combustion chamber and the surrounding engine components, which can increase thermal stress and reduce engine lifetime. Compressed intake air is often therefore cooled to increase the amount by which it can be compressed without detrimentally increasing combustion chamber temperature. This cooling can also generally increase the amount of air provided in the combustion chamber at a given intake pressure, as cool air is denser than hot air. Intake air cooking can therefore help to increase engine power.
Emission regulations are now beginning to dictate that many automotive engines mix intake air with recirculated exhaust gas, as this can reduce NOX (e.g. Nitrogen Dioxide etc.) emissions. NOX is formed in far higher quantities above certain combustion temperatures. Mixing recirculated exhaust gas with engine intake air can lower the combustion temperature and therefore reduce NOX formation. However, exhaust gas is hot. Like compressed intake air, it therefore benefits from cooling before it enters an engine's combustion chamber. In particular, cooling recirculated exhaust gas can increase the amount of exhaust gas that can be provided in the combustion chamber at a given intake pressure (e.g. improve mass flow).
Coolers for cooling compressed intake air are usually referred to as charge air coolers or intercoolers. Intercoolers can be cooled by engine coolant or other liquids, but are more commonly air cooled. An air cooled intercooler typically comprises an arrangement of tubes through which the compressed intake air can flow. For most car and light truck engines, intercoolers can provide sufficient cooling capacity without being inconveniently large and have relatively straightforward, robust and maintenance free designs. They are typically make from aluminum or plastics, as they only have to deal with relatively low temperatures (less than around 200° C.).
Coolers for cooling recirculated exhaust gas are usually referred to as Exhaust Gas Recirculation (EGR) coolers. An EGR cooler typically comprises a cylindrical shell containing one or more heat exchange tubes through which the exhaust gas can flow. Liquid coolant is passed through the shell around the tubes. The coolant is therefore in a heat exchange relationship with the exhaust gas and can cool it. Liquid cooling is used as it can typically provide greater cooling capacity than air cooling for a given heat exchange surface area. Thus, the heat exchange tubes can have a relatively large diameter and small surface area, which makes the EGR cooler tolerant to the build up of soot inside the tubes. EGR cooler are typically made from steel. One reason for this is that exhaust gas can be hot enough to damage other materials such as aluminum and plastics, but steel is more tolerant to high temperatures.
So, when it is desired to cool both engine intake air and recirculated exhaust gas, two separate coolers are conventionally provided; an intercooler and an EGR cooler. An example of such an arrangement is disclosed in patent document EP 1,138,928 A2. However, it is possible to use a combined EGR and inlet air cooler, for example as disclosed in U.S. Pat. No. 6,167,703 B1.
All such prior art systems entail a significant amount of cost, owing to the provision of conduits between the inlet and exhaust sides of the engine, and also valves to control the flow of exhaust gasses and the control systems associated with the operation of such valves. It is therefore desirable to minimize, as far as possible, the costs associated with such hardware.